1. Field of the Invention
The present disclosure generally relates to a developing agent storage device containing refill developing agent consisting substantially of toner particles and carrier particles, and an image forming apparatus having such developing agent storage device.
2. Description of the Background Art
Recently, market demand has been growing for higher image quality for images with widespread use of full-color image forming apparatuses employing electrophotography. Such demand has been met by developing smaller-sized or smaller-diameter toner particles for use in such electrophotography.
However, a drawback of such smaller-sized toner particles is that they have a proportionally greater surface area, which makes a so-called toner spent phenomenon on carrier particles more likely to occur.
Further, with growing demand for size reduction of image forming apparatuses and higher speed printing, a smaller amount of developing agent is agitated at a higher speed in a developing unit, putting greater stress on the developing agent. Such greater stress may cause abrasion of coating layer of carrier particles, or toner spent on the carrier particles, which accelerates degradation of the carrier particles. If such degraded carrier is used as the developing agent, higher image quality cannot be obtained even if smaller-diameter toner particles are used.
Typically, an image forming apparatus includes a refill unit used for refilling a developing unit with toner particles. With such a configuration, toner particles consumed in the process of developing images can be replaced with fresh toner particles refilling the developing unit at a given time interval. By contrast, however, unlike the toner particles, the carrier particles in the developing unit are not replaced in such a configuration. Accordingly, to suppress image quality degradation due to degraded carrier particles, the developing agent in the developing unit needs to be replaced at short time intervals. However, such frequent replacement of developing agent increases maintenance cost, and results in an increase of printing cost.
One background art technique to suppress such increased maintenance cost and degradation of image quality caused by carrier particle degradation employs an image forming apparatus in which the developing unit is automatically refilled with refill carrier as well as toner particles. When refilling a given amount of the refill carrier to the developing unit, a given amount of used developing agent in the developing unit is ejected from the developing unit to replace used carrier with new carrier, in which the amount of the refill carrier and the ejected amount of used developing agent may be substantially identical. With such replacement, a ratio of degraded carrier in the developing unit can be set smaller, by which degradation of image quality caused by carrier degradation in the developing unit can be reduced or prevented. Further, if a ratio of degraded carrier in the developing unit can be reduced, a replacement frequency of developing agent used in the developing unit can be reduced, by which an increase of maintenance cost can be reduced or prevented.
However, such configuration requires a carrier storage unit for storing refill carrier and a carrier refill unit in addition to a toner storage unit for storing refill toner particles and a toner refill unit, which increases both the size and the cost of the image forming apparatus.
Another background art technique an image forming apparatus that includes a developing agent storage device storing a refill developing agent, prepared by mixing refill toner particles with refill carrier particles in advance, in which a toner concentration is set higher than a toner concentration in the developing agent used in the developing unit. Such developing agent storage device may be rotated, or the developing agent in the developing agent storage device may be agitated, to supply the refill developing agent to the developing unit. When refilling a given amount of the refill developing agent to the developing unit, a given amount of used developing agent in the developing unit is ejected from the developing unit to replace used carrier with new carrier, in which the amount of the refill developing agent and the ejected amount of used developing agent may be substantially identical. Because refill toner particles and refill carrier can be stored in a same storage device in such image forming apparatus, one common storage device and one common refill unit can be used for refilling both of toner and refill carrier. Accordingly, a size increase and cost increase of an image forming apparatus can be reduced or prevented.
In addition, a toner storage unit for storing refill toner particles may be made of transformable or volume reducible material so that a storage space of used toner storage unit at a user location can be minimized and transportation costs, such as cost of recovery from a user location, can be reduced or prevented.
Yet another background art technique involves stably ejecting toner particles from such toner storage unit and effectively supplying toner particles to a developing unit in an image forming apparatus. The image forming apparatus includes a toner refill unit having a powder pump for refilling the developing unit with toner particles, in which the powder pump sucks refill toner particles from the toner storage unit by applying negative pressure to the toner storage unit to transport sucked refill toner particles to the developing unit. When the powder pump applies negative pressure to the toner storage unit, refill toner particles can be discharged from a toner exit port of the toner storage unit. Then, such discharged refill toner particles are guided to and transported to the developing unit through transport route devices by the negative pressure provided by the powder pump.
When the refill toner particles are discharged from the toner exit port by such negative pressure, refill toner particles near the toner exit port are discharged to the outside of the toner storage unit and then refill toner particles far from the toner exit port move toward the toner exit port. In such toner storage unit, discharging of refill toner particles and movement of refill toner particles can be performed by a sucking effect of the powder pump, therefore eliminating the need for the toner storage unit to have a toner transport device for moving refill toner particles.
However, when toner particles and carrier particles are separately filled in a developing agent storage device such as the aforementioned volume reducible storage unit as refill developing agent, some drawbacks occur.
For example, depending on the way in which the carrier particles are put in the developing agent storage device, the carrier particles may not be discharged from the developing agent storage device, or only carrier particles are discharged from the developing agent storage device. If carrier particles remain in the developing agent storage device without being discharged, the developing unit is not effectively refilled with the carrier particles, and thereby degradation of developing agent cannot be reduced or prevented. Further, if only carrier particles are discharged from the developing agent storage device, carrier transport by a powder pump may be stopped, in which case carrier particles clog the transport route device even if negative pressure is applied.
In light of such drawbacks of filling a developing agent storage device with toner particles and carrier particles separately, a given amount of carrier particles can be mixed in with the toner particles in advance so that the developing agent, having uniformly dispersed toner and carrier particles therewithin, can be provided to a developing agent storage device. If such mixing process is conducted for every one of the developing agent storage devices, the aforementioned irregular discharge of carrier can be reduced or prevented.
However, for reasons of manufacturing efficiency and cost, such developing agent having pre-mixed toner and carrier particles is generally prepared in bulk. However, because the toner and carrier particles have different specific gravities they cannot be uniformly mixed in bulk. As a result, multiple developing agent storage devices manufactured from even the same bulk mixture of toner and carrier can have differences or variances in the amount carrier in each of developing agent storage devices. In extreme cases, some developing agent storage devices have no carrier. Obviously, such developing agent storage device cannot supply carrier to the developing unit, which results in degradation of the developing agent.
Accordingly, a developing agent storage device that can effectively and efficiently supply carrier to a developing unit used in an image forming apparatus is desired.